diet of the american mink mustela vison and its potential .../67531/metadc102299...diet of american...

585DIET OF AMERICAN MINK IN SOUTHERN CHILERevista Chilena de Historia Natural81: 585-598, 2008

Diet of the American mink Mustela vison and its potential impact on thenative fauna of Navarino Island, Cape Horn Biosphere Reserve, Chile

Dieta del visón norteamericano Mustela vison y su impacto potencial sobre la fauna nativade Isla Navarino, Reserva de Biosfera Cabo de Hornos, Chile

ELKE SCHÜTTLER1, 2, 3, *, JAIME CÁRCAMO4 & RICARDO ROZZI2, 5

1 Department of Conservation Biology, Helmholtz Centre for Environmental Research-UFZ,Permoserstrasse 15, 04318 Leipzig, Germany

2 Omora Ethnobotanical Park (Institute of Ecology and Biodiversity-IEB, Universidad de Magallanes, and OmoraFoundation), Puerto Williams, Antarctic Province, Chile

3 Lehrstuhl für Landschaftsökologie, Technische Universität München-Weihenstephan, Am Hochanger 6, 85350 Freising,Germany

4 Instituto de la Patagonia, Universidad de Magallanes, Avenida Bulnes 01890, Punta Arenas, Chile5Department of Philosophy, University of North Texas, Denton, TX 76201, USA

* e-mail for correspondence: [email protected]

ABSTRACT

Invasive exotic species of mammalian predators represent a major cause of vertebrate animal extinctions onislands, particularly those that lack native mammalian carnivores. In 2001, the American mink (Mustelavison) was recorded for the first time on Navarino Island, in the Cape Horn Biosphere Reserve (55° S) inChile, representing the southernmost population of mink worldwide. In order to assess its potential impact onnative fauna, we studied its diet on Navarino Island, as part of an integrative management program oninvasive species. Over a three-year period (2005-2007) we collected 512 scats in semi-aquatic habitats:marine coasts, riparian and lake shores. Overall, the main prey was mammals (37 % biomass), and birds (36%), followed by fish (24 %). Over the spring and summer, mink consumed significantly more birds, whereasmammals constituted the main prey over the autumn and winter when migratory birds had left the area.Among birds, the mink preyed mainly on adult Passeriformes, followed by Anseriformes and Pelecaniformes,caught as chicks. Among mammals, the exotic muskrat (Ondatra zibethicus) was the most important prey, andtogether with the native rodent Abrothrix xanthorhinus it accounted for 78 % of the biomass intake. For anintegrated management of invasive exotic mammal species on Navarino Island and in the Cape HornBiosphere Reserve it is important to further research interactions established here among the variousintroduced mammals, and to initiate immediate control of the mink population in its initial stage of invasion.

Key words: conservation, exotic mammals, management, mustelids, waterbirds.

RESUMEN

Las especies exóticas de mamíferos carnívoros invasores constituyen una de las principales causas deextinciones de vertebrados en islas, particularmente en aquellas que carecen de predadores mamíferos nativos.En 2001, el visón norteamericano (Mustela vison) fue registrado por primera vez en Isla Navarino en la Reservade Biosfera Cabo de Hornos (55° S) en Chile, representando la población de visones más austral del mundo.Para estudiar su impacto potencial sobre la fauna nativa, estimamos su dieta en Isla Navarino como parte de unprograma de control de especies invasoras. Durante tres años (2005-2007) colectamos 512 fecas en diversoshábitats semiacuáticos: costa marina, riberas de ríos y lagos. La dieta consistió principalmente en mamíferos (37% biomasa), aves (36 %) y peces (24 %). Durante la primavera y el verano el visón consumió significativamentemás aves, sin embargo los mamíferos constituyeron la presa principal durante el otoño y el invierno, cuando lasaves migratorias han abandonado la región. La depredación sobre aves afectó principalmente a Passeriformesadultos, seguidos por Anseriformes y Pelecaniformes que fueron capturados como polluelos. Respecto a losmamíferos, un roedor exótico, la rata almizclera (Ondatra zibethicus), fue la presa principal y junto al roedornativo Abrothrix xanthorhinus constituyó el 78 % de la biomasa de presas de mamíferos. Para un manejointegral de especies de mamíferos exóticos invasores en Isla Navarino y la Reserva de Biosfera Cabo de Hornoses importante desarrollar investigación sobre las interacciones entre los mamíferos introducidos e iniciar uncontrol inmediato de la población de visones en su estado inicial de invasión.

Palabras clave: conservación, mamíferos exóticos, manejo, mustélidos, aves acuáticas.

586 SCHÜTTLER ET AL.

INTRODUCTION

Invasive exotic species and their impacts arecurrently regarded as one of the major causesof anthropogenic global change (Sala et al.2000) and biodiversity loss (Vitousek et al.1997). In particular, invasive predators canhave severe impacts on native prey populations,especially on remote islands due to preynaivety or a lack of their natural predators orcompetitors that would have otherwise limitedtheir success (Elton 1958, Macdonald & Thom2001). In the case of evolutionary isolation ofnative species, introductions of carnivorespecies to island ecosystems can even lead tolocal extinctions, as it has been documented forseveral bird species (Courchamp et al. 2003).

The American mink (Mustela visonSchreber, 1777) is a successful alien predator inmost European countries, where it hasestablished feral populations following itsintroduction from North America for thepurpose of fur farming (reviews in Macdonald& Harrington 2003, Bonesi & Palazon 2007).In South America the mink was introduced toChile and Argentina in the 1930s (Jaksic et al.2002), where populations established insouthern Chile (Sandoval 1994, Rozzi &Sherriffs 2003, Anderson et al. 2006a) andArgentina (Pagnoni et al. 1986). Mink aremedium-sized mustelids with a body weight ofabout 1 kg. They are semi-aquatic mammalsfound associated with marine shore habitats,river banks, lake shores, freshwater andsaltwater marshes. As generalist predators theirdiet includes prey from both aquatic andterrestrial sources in variable proportions andstrongly reflects local and seasonal availabilityof prey (Dunstone 1993).

Being a highly adaptable and opportunisticpredator various studies in Europe have shownthat mink can be detrimental to native species(Macdonald & Harrington 2003). The mostapparent impact of the mink is a reduction inthe range or population size of native prey ashas been well documented for seabird colonieson islands (Clode & Macdonald 2002,Nordström et al. 2004), ground-nesting inlandwater birds (Ferreras & Macdonald 1999),intertidal marine communities (Delibes et al.2004), rodents (Jefferies 2003) and amphibians(Ahola et al. 2006). Negative impacts due tocompetition are subject to discussion for the

European mink (Maran et al. 1998, Sidorovichet al. 2001). In South America mink havecaused reductions in waterbird populations(Lizzaralde & Escobar 2000). They are alsoconsidered to be responsible for the decline ofthe river otter Lontra provocax (Thomas, 1908)(Previtali et al. 1998), although Medina (1997)found little support on competition for spaceand food.

The case of Navarino Island represents arecent invasion of the mink, where it was firstrecorded in 2001 (Rozzi & Sherriffs 2003).Navarino Island is located south of Tierra delFuego, from which it is separated by the BeagleChannel. At numerous points this channel isless than 5 km wide. Mink that escaped or werereleased from mink farms on Tierra del Fuegomight have swum across the Beagle Channel,reaching Navarino Island, and other islands ofthe Cape Horn Biosphere Reserve (Rozzi &Sherriffs 2003). Here, they represent a newguild because Navarino Island lacks nativeterrestrial mammalian predators. Thus amongthe mechanisms by which introducedcarnivores affect the local biota (competition,disease, interbreeding, predation, Macdonald &Thom 2001) predation is the most expectedmechanism for mink on Navarino, withpossible indirect effects on trophic webs.

On Navarino Island the most diverse andabundant group of vertebrates are birds (Rozziet al. 2006a). Many of them are ground-nesting,and expected to be especially vulnerable topredation from introduced mink as behavioraladaptations to terrestrial predators might lack(Anderson et al. 2006a, Soto & Cabello 2007).Although the Cape Horn region is part of oneof the most pristine areas in the world(Mittermeier et al. 2003), this archipelago hasnot only been invaded by mink, but also byferal domestic animals, rodents and two moreNorth American wild fur mammals: the beaver(Castor Canadensis Kuhl, 1820) and themuskrat (Ondatra zibethicus Linnaeus, 1766).In total, the assemblage of exotic terrestrialmammals on the island outnumbers their nativecounterparts (Anderson et al. 2006a). Some ofthese exotic species of rodents might representprey for the mink, thereby generating possiblepredator-prey interactions among exoticspecies.

The prime purpose of this study is toquantify the composition of the diet of the

587DIET OF AMERICAN MINK IN SOUTHERN CHILE

southernmost population of American mink inits initial stage of colonization, consideringseasonal and habitat variations, in order toprovide an initial baseline diagnosis about itspotential impact on native and exotic fauna ofNavarino Island, and the Cape Horn BiosphereReserve. Our hypotheses are as follows: First,given that birds are the most abundant group ofterrestrial vertebrates on Navarino, birds couldrepresent the main prey group in the diet of themink. Second, given that in Cape Horn severalbird species and/or populations are wintermigrants, we expect birds to be the main preyespecially during the breeding season and inmarine coastal habitats where bird populationsare particularly abundant and diverse. Third,given that the exotic muskrat is an importantprey for mink in its original distribution range;this rodent species could also constitute animportant prey for mink populations in the areaof Cape Horn, thus facilitating the arrival of thenewcomer mink (i.e., invasional meltdownhypothesis by Simberloff & Von Holle 1999).We will try to estimate the absolute number ofbirds, mammals, and fish that mink removemonthly and discuss these results in the contextof conservation and management with a specialfocus on the potential impact that the recentlyintroduced mink might have on native bird,especially ground-nesting, and mammalpopulations.

MATERIAL AND METHODS

Study area

The study was carried out on Navarino Island(2528 km2), located at the extreme southern tipof South America (Fig. 1). The island formspart of the Cape Horn Biosphere Reserve (54°-56° S) and belongs to the Magellanic Sub-Antarctic Evergreen Rainforest ecoregion,recently identified as one of the 24 mostpristine wilderness areas of the world(Mittermeier et al. 2003). The main habitatsinclude (i) evergreen and deciduous forestsdominated by the genus Nothofagus , (ii)peatlands, moorlands, and bogs, (iii) alpinecommunities dominated by cushion plants andlichens, (iv) streams and lakes, and (v) thicketsor scrublands in naturally or anthropogenicallydisturbed areas (Pisano 1977, Rozzi et al.

2006b). The climate is oceanic, with a lowannual thermic fluctuation (< 5 °C), a meanannual temperature of 6 °C, and an annualprecipitation of 467.3 mm (Pisano 1977).During the winter, streams and lakes are ice-bound. The human population is concentratedin the town of Puerto Williams, the capital cityof the Chilean Antarctic Province, on thenorthern coast of Navarino. Access to the town,rural settlements and navy stations reliesmostly on access from the sea, except for a dirtroad that connects the entire northern coast ofNavarino Island.

Potential vertebrate prey on Navarino Island

With 154 species, birds represent the mostabundant and diverse group of terrestrialvertebrates on the island (Couve & Vidal2003, Rozzi et al. 2006a). Native mammalsinclude only five species: one Artiodactyla,two Chiroptera, and two Rodentia. At present,exotic mammals include eleven species,therefore outnumbering native species. Theyinclude four species of rodents: Castorcanadensis, Mus musculus (Linnaeus, 1758),Ondatra zibethicus, and Rattus norvegicus(Berkenhout, 1769) (Anderson et al. 2006a).Ondatra zibethicus is an important native preyto mink in North America (Dunstone 1993).As far as freshwater fish are concerned,Navarino Island only hosts one commonnative species (Galaxias maculatus Jenyns,1842), two extremely rare native species(Applochiton spp.) and two exotic species oftrout, Salvelinus fontinalis (Mitchill, 1814),and Oncorhynchus mykiss (Walbaum, 1792)(Moorman 2007). However, the marine fishfauna in the Beagle Channel is rich andincludes more than 50 species (López et al.1996). There are no amphibians or reptilespresent on Navarino Island (Anderson et al.2006a).

Dietary analysis

Diet was analyzed by examining 512 feces (=scats) collected from 36 sites on NavarinoIsland between April 2005 and March 2007.The sites included different semi-aquatichabitats: 13 marine coastal sites, 14 sites alonglake shores, and nine sites along riparianshores. The majority of the sites (28 sites) were


located at the northern margin of NavarinoIsland due to the limited accessibility of thesouthern areas. To ensure that collected scatsrepresented different individuals we sampledsites that were separated by at least three km(the average linear territory size described forthe American mink, Dunstone 1993). Wesearched for scats at all sites over the summer.Due to climatic conditions and accessibility, weonly conducted scat searches over the spring,autumn, and winter seasons at coastal sites. Allscats were frozen for long-term storage.

Collected scats were thawn and soaked inwater overnight prior to sieving (0.3 mm). Thewashed scats were then dried at 50 °C (24 h) andstored in paper bags. We sorted the undigestedprey remains into six categories (mammals,birds, fish, insects, crustaceans, mollusks) usinga binocular microscope. Seeds and plant materialwere excluded from the analysis as we suspectedthat this was a result of secondary prey,accidental intake or adhesion after defecation.We estimated the percentage volume of eachprey category per scat to the nearest 10 % andweighed it to 0.01 g.

For the identification of mammals and birdswe used the reference collections of theInstituto de la Patagonia, Universidad deMagallanes, complemented by our ownadditional collections, and local keys (Reise1973, Chehébar & Martín 1989, Reyes 1992,Rau & Martínez 2004). Mammals wereidentified to the species level by examininghair samples; birds were identified to the orderlevel, the taxonomic level a microscopicexamination of feathers permits (Day 1966). Toaddress the question of which age categorybirds are preferably consumed by mink weclassified birds that had been identifiedtaxonomically into adults and chicks. Weassigned the sample to chicks if at least three ofthe following features applied: long slenderbarbs at short rhachis, presence of papillae,truncated feather shape, lack of coloration, lackof pennaceous barbs, and a small amount ofbones in the sample (Ewart 1921, Busching2005). Insects were identified by a localentomologist to the species level, and wherethis was impossible to the next highertaxonomic level (order, subclass).

Fig. 1: Map of southern South America with the study area Navarino Island south of theArgentinean portion of Tierra del Fuego (separated by the Beagle Channel).Mapa del sur de Sudamérica con el área de estudio Isla Navarino al sur de Tierra del Fuego Argentina (separado por elcanal Beagle).


To quantify the relative contribution of preygroups to mink’s diet we applied three indices;the first two are commonly used (Jedrzejewska& Jedrzejewski 1998):

(1) Percentage of relative frequency ofoccurrence of each food item (RFO), calculatedas the number of occurrences of a preycategory divided by the number of occurrencesof all prey categories. With this index, smallprey items tend to be overrated in terms ofimportance.

(2) Percentage biomass of a given prey item(BIO) estimated by multiplying its dry mass byits empirically determined coefficients ofdigestibility. This coefficient is measured as theratio of fresh mass of a given prey to the drymass of its remains in scats (Jedrzejewska et al.2001). We applied the following correctionfactors: mammals (17.3), birds (17.2), eggs(687.5), fish (30.8), crustaceans and mollusks(14.8), provided by Brzezinski & Marzec(2003). For insects we used the value (5.0)provided by Lockie (1961). Calculating thebiomass reflects the real intake of prey as ittakes into account different sizes anddigestibility of prey (Brzezinski & Marzec2003).

(3) Percentage occurrence of the dominantitem (POD) with the item constituting thelargest volume class considered as thedominant one (Hammershøj et al. 2004). Thismethod can compensate for the disadvantage ofsecondary prey intake.

Statistics

We conducted Spearman’s rank correlationsbetween the corresponding prey groups ofdifferent indices (RFO, BIO, POD) in order toevaluate whether those indices were comparablewith each other. We assessed differences in thediet composition between habitats and seasonsusing chi-squared tests with Yates’ continuityand Bonferroni corrections, Fisher’s exact testswhen expected values were less than five, andtwo-sample tests for equality of proportions, alltwo-sided, running the R version 2.7.1. (RDevelopment Core Team 2008).

In order to measure the degree ofspecialization of mink in different habitatsand seasons we calculated its food-nichebreadth for the six defined food categoriesusing Hurlbert’s standardization of Levins’ B

index (Levins 1968, Hurlbert 1978, Krebs1999):

Bs =

with Bs = standardized niche breadth, B =Levins’ index, n = number of food categories,

B =

where pi is the proportion of items in the dietthat are of food category i. The proportion wascalculated for each of the three indices used inthis study (RFO, BIO, POD). The range ofLevins’ B index is 1 to n. Therefore it dependson the number of food categories. Thestandardized niche breadth Bs is independent ofn and has a range from 0 to 1, with 1 indicatingthe broadest niche.

We estimated the intake of prey groups bythe mink in order to evaluate its impact on preypopulations where abundances are known. Toassess how many birds, mammals, and fish asingle mink might consume during a warmmonth, we followed the approach developed byJedrzejewska & Jedrzejewski (1998) inBartoszewicz & Zalewski (2003). Accordingly,the number of a given prey group eaten per dayper mink (Npd) was calculated as:

Npd =

where DFC is the average daily foodconsumption of mink (app. 190 g, 153 ± 48 gfor females, 231 ± 72 g for males, estimatedfrom Dunstone 1993), Bp is the fraction ofgiven prey biomass in the mink diet, and Wtp isthe wet mass of a given prey: 75 g for smallbirds, 35 g for small rodents, 1100 g for themuskrat, and 113 g for fish (Dunstone 1993,Bartoszewicz & Zalewski 2003). We used anoverall index of 75 g for birds as our resultsindicated that bigger birds (e.g., Anseriformes)were almost exclusively consumed as chicks.

RESULTS

The analysis of scat content showed that themink diet consisted mainly of mammals andbirds (Fig. 2). Combined, both taxonomicgroups accounted for more than half of the

(B – 1)(n – 1)

1∑ pi

2

DFC × Bp

Wtp


mink diet using the relative frequency ofoccurrences index (59.3 % RFO), for 68.0 %when estimated by the biomass index (BIO),and for 80.1 % with the dominant item method(POD). In the overall diet, mammals were moreimportant than birds using RFO (χ2 = 7.79, df =1, P = 0.005), but not significantly differentform birds in biomass intake (χ2 = 0.73, P =0.394). Fish were the third most importantcomponent in the mink diet accounting for14.1-23.7 %. Overall, we found that terrestrialfood (72.5-83.2 % mammals, birds, insects)provided a much more important source of foodfor mink than did aquatic prey (16.8-27.5 %fish, crustaceans, mollusks) (for all indices χ2-values ≥ 375.35, all df = 1, all P-values <0.0001).

Variations in mink diet among habitats

All three indices correlated significantly witheach other (Spearman rank correlations, all n =6 prey groups: RFO with BIO, rs = 0.94, P =0.017; RFO with POD, rs = 1.0, P = 0.003; BIOwith POD, rs = 0.94, P = 0.017). For thisreason, we focused on the most conservative

index, the dominant item method (POD), toevaluate differences in prey compositionbetween scats collected in different habitattypes. Mink diet in the summer variedsignificantly among different habitat types (Fig.3). The dominant item index differed betweenriparian and marine coastal habitats (Fisher’sexact test, df = 5, P < 0.0001), and riparian andlake habitats (df = 5, P < 0.0001). Differencesbetween marine coastline and lakes were alsosignificant (df = 5, P = 0.031), but the contrastin values was less pronounced. In lake habitats,the proportion of birds (58.0 %) exceeded theproportion of mammals (34.1 %) significantly(χ2 = 9.15, df = 1, P = 0.002). In contrast, inriparian habitats bird prey was much lessfrequent than mammal ones (22.4 versus 55.1%, χ2 = 9.67, P = 0.002). Indeed, in riparianhabitats the consumption of birds by the minkwas significantly lower than in marine (χ2 =6.62, P = 0.01) and lake habitats (χ2 = 10.67, P= 0.001). Regarding the other taxonomicgroups, fish was only found to be frequent preyin marine coastal habitats (17.4 %). Thus theconsumption of fish by the mink wassignificantly higher in marine habitats

Fig. 2: Overall occurrence of prey categories in scats of American mink on Navarino Island.Percentages are based on data from all scats (n = 512) collected from three types of semi-aquatichabitats (marine, riparian, lake) over the four seasons during a three-year period (2005-2007); RFO= relative frequency of occurrences of each prey category, BIO = percentage of biomass consumed,POD = percentage of occurrence of each prey category as the dominant itemPresencia total de categorías de presa en heces del visón en Isla Navarino. Los porcentajes están basados en datos de todaslas heces (n = 512) recolectadas en tres diferentes tipos de hábitats semiacuáticos (costa marina, riberas de ríos y lagos) enlas cuatro estaciones durante un período de tres años (2005-2007); RFO = frecuencia relativa de presencia de cadacategoría de presa, BIO = porcentaje de biomasa consumida, POD = porcentaje de presencia de cada categoría de presacomo ítem dominante.


compared to lake (χ2 = 5.78, P = 0.016), andriparian habitats (χ2 = 6.19, P = 0.013).Regarding other prey (insects, crustaceans,mollusks), insects were found to be thedominant item in riparian habitats occurring in20.4 % of the analyzed scats, while crustaceanswere the most important prey in marine coastalhabitat accounting for 5.2 %.

Variations in mink diet among seasons

We assessed seasonal differences in the diet ofthe mink in marine coastal habitats focusing onthe POD index as done for differences betweenhabitats. Diet composition between spring-summer and autumn-winter, respectively, wasinsignificant (Fisher’s exact test, df = 5, P =

Fig. 3: Differences in the consumption of mam-mals, birds, fish, and other prey (insects, crus-taceans, mollusks) by mink over the summer indifferent habitat types (n = 292 scats). For eachgraph different letters above the bars indicatesignificant differences (P < 0.05 with 2-sampletests for equality of proportions) in the percen-tage of occurrence of the dominant item (POD)in a prey group among habitat types.Diferencias en el consumo de mamíferos, aves, peces yotra presa (insectos, crustáceos, moluscos) por el visón du-rante el verano en diferentes tipos de hábitats (n = 292heces). Hábitats con distintas letras indican diferenciassignificantes (prueba de igualdad de proporciones, P <0,05) en el porcentaje de presencia del ítem dominante(POD) en una categoría de presa.

Fig. 4: Differences in the consumption of mam-mals, birds, fish, and other prey (insects, crus-taceans, mollusks) by mink during warm(spring-summer) and cool (autumn-winter) sea-sons in marine coastal habitat (n = 375 scats).For each graph different letters above the barsindicate significant differences (P < 0.05 with2-sample tests for equality of proportions) inthe percentage of occurrence of the dominantitem (POD) in a prey group among seasons.Diferencias en el consumo de mamíferos, aves, peces yotra presa (insectos, crustáceos, moluscos) por el visón du-rante la temporada cálida (primavera-verano) y la tempora-da fría (otoño-invierno) en hábitat marino (n = 292 heces).Temporadas con distintas letras indican diferencias signifi-cantes (test de igualdad de proporciones, P < 0,05) en elporcentaje de presencia del ítem dominante (POD) en unacategoría de presa.

0.119, and P = 0.233, respectively) so that datawere pooled for the warm season (spring-summer) and the cool season (autumn-winter).However, the diet of the mink in warm and coolseasons varied significantly (df = 5, P <0.0001) (Fig. 4). Scats collected during thewarm season at marine coastal sites wereapproximately equally dominated by mammalsand birds (37.6 versus 40.7 %, χ2 = 0.4, df = 1,P = 0.528). In contrast, during the cool season,scats collected in marine habitats weredominated by mammal items (59.8 % POD),while birds, which were dominant items in only16.2 % of analyzed scats, were significantly


less consumed (χ2 = 45.33, df = 1, P < 0.0001).Fish represented the dominant item in 16.7-19.7 % of scats in warm and cool seasons, andno significant temporal variation could bedetermined. Furthermore, no significantseasonal fluctuations for the other prey groupscould be found, which made up less than 5 %of the dominant items in warm and coolseasons.

Trophic niche breadth

The standardized trophic niche breadths duringthe summer season were highest for minkinhabiting marine coastal habitats (Table 1).Lake habitats exhibited low values of trophicniche breadth. Mink diet in lake habitatsconcentrated on birds (58 % POD); fish andother prey groups were almost absent (Fig. 3). Inmarine habitats, mink diet showed the highestvariation during summer, and the lowest duringwinter when mink relied essentially on mammals(63.6 % POD), and entire prey groups (insects,crustaceans, mollusks) lacked in its diet. Forspring and autumn the trophic niche breadthswere intermediate.

Taxonomic identification

We identified prey groups from 193 scatscollected during the warm season (spring-summer) in marine coastal habitat. We focusedon this sample as food niche breadth wasbroadest in marine habitats, and in order tocoincide with the breeding season of birds. Birdswere the most diverse group of prey on NavarinoIsland. Seven of the twelve orders of birds

breeding on the island were present in the scatsanalyzed (Table 2). Passeriformes were the mostabundant order in the diet, with indices from10.2-14.8 % (RFO, BIO, POD). However,Pelecaniformes, Anseriformes, and eggs alsoplayed an important role in terms of biomassintake. Together these four groups accounted for76.2 % of the bird biomass consumed by mink.Passeriformes were mainly caught as adults,whereas all other bird orders were caught aschicks, with the difference being highlysignificant (χ2 = 37.9, df = 1, P < 0.0001).

As far as mammals are concerned, the twodominant species were the native rodentAbrothrix xanthorhinus (Waterhouse, 1837)and the exotic muskrat (Ondatra zibethicus),with 8.9-14.8 %, and 8.7-15.2 % (RFO, BIO,POD), respectively (Table 2). Together, A.xanthorhinus and O. zibethicus represented78.2 % of the mammal biomass consumed bythe mink on Navarino Island. The relativeimportance of Oligoryzomys longicaudatus(Bennett, 1832), the second native rodentspecies on the island, was modest. Its relativefrequency of occurrence, biomass anddominance as a prey item were less than a thirdof the values determined for A. xanthorhinusand O. zibethicus. The presence of the housemouse (Mus musculus) in the diet wasnegligible. It was found only in three samplescollected in the vicinity of houses. Noticeably,the two other exotic rodent species, namely thebeaver (Castor canadensis) and the Norway rat(Rattus norvegicus), were not found in any ofthe scats collected in marine coastal habitatsover the spring and summer. The occurrence ofmink hair in some of the scats was ascribed to

TABLE 1

Levins’ standardized niche breadth for mink on Navarino Island, estimated on the basis of scatscollected in marine coastal, riparian, and lake habitats over the summer (n = 292), and for scats

collected from marine habitats over different seasons of the year (n = 375)

Amplitud de nicho trófico estandardizado de Levins para el visón en Isla Navarino, estimado a base de heces recolectadasen costa marina, riberas de ríos y lagos durante el verano (n = 292), y de heces recolectadas en hábitats marinos durante

diferentes estaciones del año (n = 375)

Index Habitat (summer) Season (marine coast)

Marine Rivers Lakes Spring Summer Autumn Winter

RFO 0.78 0.45 0.47 0.64 0.78 0.52 0.35

BIO 0.43 0.18 0.17 0.39 0.43 0.31 0.40

POD 0.42 0.30 0.24 0.35 0.42 0.28 0.21


grooming. In summary, the proportion of exoticmammals in terms of biomass was 49.9 %, aproportion that exceeds the value of 43.2 %estimated for native mammals (the other 6.9 %correspond to non identified mammal prey).The contribution of invertebrates to the mink’sbiomass intake (2.9 %) was negligible (Table

TABLE 2

Diet composition of American mink over thewarm season (spring-summer) in marine coastalhabitat on Navarino Island (n = 193 scats, 2005-2007); RFO = relative frequency of occurrences,BIO = percentage of biomass, POD = percentage

of occurrence of dominant item

Composición de la dieta del visón durante la temporadacálida (primavera-verano) en hábitat marino costero en

Isla Navarino (n = 193 heces, 2005-2007); RFO =frecuencia relativa de presencia, BIO = porcentaje

biomasa, POD = porcentaje presencia del ítem dominante

Prey RFO BIO POD

Abrothrix xanthorhinus 8.9 10.1 14.8Mus musculus 0.6 0.1 0.3Mustela vison 2.4 1.3 3.8Oligoryzomys longicaudatus 2.6 3.2 4.8Ondatra zibethicus 8.7 13.9 15.2Unidentified mammals 6.1 2.1 2.8

Total mammals 29.2 30.8 41.7

Anseriformes 3.9 5.1 6.9Ciconiformes 1.3 1.7 2.4Coraciiformes 0.4 0.3 0.7Gruiformes 0.2 0.4 0.3Passeriformes 10.2 11.3 14.8Pelecaniformes 3.7 7.5 6.2Podicipediformes 0.9 1.0 1.7Eggs 3.9 6.6 -Unidentified birds 6.3 6.2 6.6

Total birds 30.7 40.0 39.7

Fish 15.5 26.3 14.8Arachnida, Acari 0.4 - -Coleoptera 0.9 - -Hemiptera, Heteroptera 0.2 - -Aegorhinus vitulus 0.2 - -Microplophorus magellanicus 1.5 0.1 0.3Pycnosiphorus femoralis 1.7 0.1 0.3Sinopla perpunctatus 0.2 - -Unidentified insects 5.7 - 0.3

Total insects 10.7 0.3 1.0

Crustaceans 10.9 2.4 2.8Mollusks 3.0 0.2 -

Food niche breadth 0.68 0.42 0.36Total N items or biomass (g) 541 2,566.6 290

2). In spite of this however, mink seem toactively search for insects because all of theinsect taxa identified, except for the coleopteraMicroplophorus magellanicus (Blanchard,1851), were flightless, and therewith easier tocatch. Hence, insects were probably consumeddirectly by mink (although we cannot excludeconsumption by other active hunters).

Monthly food consumption of mink

How hungry are mink? On average the dailyfood consumption of a mink is approximately190 g (estimated from Dunstone 1993).Following our biomass fractions (BIO) of preygroups over the spring and summer we canestimate an extrapolated monthly (30 days)consumption, which does not take into accountvarying energy demands of the animal.Accordingly, a single mink would consumemonthly on average: nine Passeriformes, fourAnseriformes, six Pelecaniformes, 13 otherbirds (total birds: 32), 17 Abrothrixxanthorhinus , f ive Oligoryzomyslongicaudatus, one muskrat, three other smallrodents (total mammals: 26), and 13 fish. Thesmall proportion of remaining prey groups (2.9% BIO) was neglected in this estimation. Basedon trapping data of mink on Navarino Island,mink relative densities ranged from 0.79-1.32individuals km-1 along marine coastal habitat(Anderson et al. 2006a). Applying thesedensities to a 10 km stretch of coastline eight tothirteen mink living there could roughlyconsume about 248-415 small birds, 205-343mammals, and 105-175 fish in one monthduring the warm season.

DISCUSSION

Mammals as a reliable source, birds welcomewhen available

Diet composition showed that mink reliedprincipally on mammalian and avian prey withsignificant variations over habitats and seasons.In marine coastal habitat and in particular atlakes birds were the principal prey group. Overthe warm season mink preyed on bothvertebrate groups almost equally, but reliedmainly on mammals over the cool season.These results suggest that mammals represent a


stable base in the diet compensating for periodswith lower bird availability.

Compared to other studies on mink dietworldwide, the proportion of birds in scatsfrom marine coastal habitat on Navarino Islandover the summer was exceptionally high.Further North in Argentinean Patagonia coastalmink predominantly prey on crustaceans andinsects (Previtali et al. 1998). In Eurasia fishand crustaceans seem to be the most importantprey for mink in marine habitat (Jedrzejewskaet al. 2001, Delibes et al. 2004), althoughmammals can also play an important role(Dunstone & Birks 1987). In riparian habitats,fish, mammals, and amphibians are reported asprincipal prey groups in the diet of introducedmink (Medina 1997, Ferreras & Macdonald1999, Jedrzejewska et al. 2001, Hammershøj etal. 2004). Noticeably, few studies report a highconsumption of birds, which only occurred inproductive waterbird breeding habitats andareas with large wildfowl populations in inlandwetlands, especially during the warm season(Ward et al. 1986, Arnold & Fritzell 1987,Bartoszewicz & Zalewski 2003). Therefore, theopportunistic diet habits of the mink (Dunstone1993) seem to reflect the abundance anddiversity of prey available in each region.

The exceptionally high abundance of birdsin the diet of mink at the southern end of theAmericas might be explained by at least threefactors: (i) in the insular austral environmentamphibians and reptiles as possible prey groupsare absent (ii) aquatic prey might play a minorrole due to the climate conditions incombination with the greater level of energyexpenditure used to catch it (Stephenson et al.1988), and (iii) during the warm season, thedensity of birds returning from migration ishigh, the mobility of birds during incubation,brood rearing or moulting is reduced(Bartoszewicz & Zalewski 2003), and manyground-nesting birds on Navarino supplyvulnerable offspring.

Impact on native prey populations

In order to assess the direct ecological impactof mink on prey populations we must identify“sensitive species”, i.e. species with lowabundances, low hunting effort for mink, andhigh energy content (Dunstone 1993). Ouridentification of bird remains in mink scats is

reliable to the level of orders. Consequently,we attempted to identify potentially sensitivespecies among the main orders we found in thediet of mink on Navarino Island. Passeriformeswere the most abundant bird order in the diet,and furthermore affected as being caught asadults, i.e. as individuals contributing to thereproduction of the population. Although wedon’t know whether these passerines wereground or tree-nesting, we suggest thatpotentially sensitive species might be those thatbreed in open nests at low height, and presentlow abundances in the Cape Horn archipelagoregion. Based on avian censuses for NavarinoIsland (Anderson & Rozzi 2000, Anderson etal. 2002, McGehee et al. 2004) these criteriatarget three species: the ground nestingMagellanic tapaculo (Scytalopus magellanicusGmelin, 1789), the Patagonian tyrant(Colorhamphus parvirostris Darwin, 1839),and in particular the fire-eyed diucon (Xolmispyrope Kitt l i tz, 1830) (Ippi personalcommunication). In inland wetlands birdspecies that are locally rare, such as theFuegian snipe (Gallinago stricklandii Gray,1845) (Couve & Vidal 2003) might bethreatened because in comparison to marinecoastal habitats, inland wetlands on NavarinoIsland seem to harbor less potential prey formink (lack of crustaceans, marine fish etc.).

With respect to large ground-nesting birds(e.g. Anseriformes, Pelecaniformes) our resultsconfirm that mink predominantly caught theirchicks. Among Anseriformes, two speciesendemic to the subantarctic archipelago regionof southwestern South America (41-56° S,including the Falklands) have small populationsthat might be particularly vulnerable to minkpredation of their offsprings: the flightlesssteamer duck (Tachyeres pteneres Forster,1844) and the kelp goose (Chloephaga hybridaMolina, 1782). Both species are strictly coastal,and they inhabit the area all year round (Couve& Vidal 2003). Their population densitiesalong the coasts of the Beagle Channel (0.74,and 0.46 birds km-1 for T. pteneres and C.hybrida, respectively, estimated by Raya &Schiavini 2002) are very low in the light of thenumbers of birds consumed by mink in ournumeric example (25-42 month-1 km-1).

Regarding native mammals, mink huntednative rodents in proportion to theiravailability. In the spring-summer diet we


found three times more Abrothrix xanthorhinusthan Oligoryzomys longicaudatus . Thisproportion resembles the relative abundancesobtained for these two species through censuseswith Sherman traps on the study sites onNavarino Island (Gañan et al. unpublishedresults). A. xanthorhinus was trapped six timesmore frequently than O. longicaudatus. For A.xanthorhinus Gañan et al. estimated meanrelative densities of 8-39 individuals ha-1 (per100 trap nights, depending on the habitat type:forest, pasture, reed, shrubland), and 0-6individuals ha-1 for O. longicaudatus ,respectively, during spring to autumn (n =3,800 trap nights). Hence, based on ourestimations for the consumption of nativerodents from our biomass fractions in mink diet(17-29 month-1 km-1), it seems that Mustelavison does not represent a threat to thesemammal species, although these roughestimates have to be treated with caution. Theindirect impact of mink on changes in the preyoffer for autochthonous bird predators requiresfurther analysis.

Exotic mammals as prey for mink

Our results from marine coastal scats over thewarm season show a considerable proportion ofmuskrat among mammal biomass consumed bymink on Navarino Island. Interestingly, thisresult coincides with studies from NorthAmerica, where muskrats are the largest andprobably most important mammalian prey formink (Dunstone 1993). Over the winter monthsin particular young muskrat are vulnerable tomink predation (Errington 1954). Thus,populations of these two North Americaninvasive mammals have reestablished theirpredator-prey interactions at the southern endof the continent. We lack systematic abundancedata of the muskrat, but SAG’s ControlProgram of Exotic Species in Magallanes (Soto& Cabello 2007), reports that 250 muskrat werecaptured in Tierra del Fuego and the Cape HornRegion by a single trapper working in lake andriparian habitats over 2005/2006. We thereforeassume that muskrat is a reliable and highlyenergetic source for mink. This result adds onemore example of facilitatory interactionsbetween an invasive species already present,the muskrat (arrival in the fifties, Jaksic et al.2002), aiding a new species to establish

(Simberloff & Von Holle 1999), although notin a mutual way.

As far as the other species of NorthAmerican introduced rodents are concerned, wedid not find any beaver remains in the diet ofmink in marine habitats. This might be due tothe fact that beavers are mostly associated withriparian habitats (Anderson et al. 2006b).Studies on mink scats (n = 235) collected ininland wetlands on Navarino Island have shownthat only 2 % contained beaver remains (Ibarra2007). Again, these results for Navarino Islandcoincide with those found in North America(Dunstone 1993) and Europe (e.g. Brezezinski& Zurowski 1992) where beavers are notreported as prey species. A probableexplanation is that beavers are too large as preyfor mink (their body weight ranges from 18-23kg, Aleksiuk 1968), and that young beavers aresufficiently guarded by their parents(Brezezinski & Zurowski 1992). However,mink might profit from the coexistence withbeavers, since their engineering activitiesprovide denning facili t ies for the mink(Zurowski & Kammler 1987).

Concluding remarks

Birds constituted an important prey group for theintroduced mink on Navarino Island, especiallyin marine coastal habitat and lakes during thebreeding season. Studies in other insularecosystems have shown that the introduction ofmink can lead to severe reductions of birdpopulations (Ferreras & Macdonald 1999,Nordström & Korpimäki 2003). We identifiedtwo Anseriformes (Tachyeres pteneres andChloephaga hybrida) endemic to southern SouthAmerica, which might be seriously threatenedby the presence of mink on Navarino Island. Forthese reasons we advise governmental agencies(Iriarte et al. 2005) to start controlling minkpopulations immediately. In order to increasetrapping success, control should take placemainly during periods of high activity of mink(spring and autumn, Moore et al. 2003), andwith particular intensity at nesting habitats ofsensitive bird species during their breedingperiods (under the consideration of the trapper’sinfluence).

For an integrated management of exoticspecies on Navarino Island and the Cape HornBiosphere Reserve it is important to undertake


further research on the direct and indirectinteractions established here among the variousintroduced mammal species (Silva & Saavedra2008). The muskrat might protect nativespecies from being preyed upon theopportunistic predator, but on the other hand itrepresents a reliable food source for the mink.Finally, we recommend that long-termmonitoring of bird populations shouldaccompany the mink management strategiesthat are currently being implemented in thisremote region of the world. The critical needfor conserving the avifauna and ecosystemintegrity of the Cape Horn Biosphere Reserveis especially relevant for ecotourism, whichcurrently represents the main option forachieving both economic and environmentalsustainability at the southern end of theAmericas (Rozzi et al. 2004).

ACKNOWLEDGMENTS

We thank José Llaipén for his great support infield work and laboratory analysis. StevenMcGehee and Melisa Gañan assisted with thefield work. We are grateful to Eduardo Faúndezwho identified the insects. Kurt Jax and JorgeGibbons provided helpful advice regardingscientific questions. We thank Carlos Soto andAlex Muñoz for hosting us in their laboratories.Jana Zschille provided us with experience andbibliographic citations. We express ourgratitude to two anonymous reviewers whosecomments greatly improved the paper. Themanuscript also benefited due to commentsfrom Klaus Henle, Kurt Jax, Steven McGehee,Michael Gerisch, Simone Lampa, and BiancaBauch. This work was co-sponsored by theGerman Academic Exchange Service (DAAD),by the Chilean Millennium Institute of Ecologyand Biodiversity Contract (ICM, PO2-051-F),the Universidad de Magallanes, and the OmoraFoundation.

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